The present invention relates to a technique wherein mobile stations communicate with other stations using spread spectrum communication; and, more particularly, the invention relates to mobile stations equipped with rake receiving circuits, such as digital portable telephones or mobile telephones of CDMA (Code Division Multiple Access) system.
For spread spectrum communication employed in a CDMA system, information signals are modulated by spectrum spreading using a spread code, which is called a pilot PN (Psuedo Noise) code, and the modulated signals are then transmitted from a base station. A mobile station, such as a portable telephone, reproduces the original information signal by demodulating the received signals using the same PN sequence which was used at the base station to de-spread the modulated information signals.
On the other hand, the radio waves, which are received at such a mobile station, are generally multipath waves, because the received waves are formed by a radio wave that has traveled via various routes after being reflected and diffracted by a building and other objects, for example. Such a multipath signal is characterized by a mutual interference between the phases of the waves forming the received signal, and this causes the problem of a reduction in the gain of a reduction in the ratio of received signal strength S and the magnitude of the noise N (S/N ratio).
To improve the S/N ratio in this type of system, a path diversity system using rake receiving is proposed in the Japanese patent Laid-open Publication 7-231278 official gazette. In such a system, several de-spreading circuits and demodulation circuits are provided according to the number of multipaths, and multipaths on which the propagation time is greater than the unit time of a spreading chip are decomposed, so that each de-spreading circuit performs de-spreading. That is, the multipath processing is assigned to several de-spreading circuits, and each de-spreading circuit performs de-spreading processing by using replica PN sequence codes for the phases that correspond to the assigned paths. Each demodulation circuit demodulates several input signals that are separated and de-spreading processed, and the demodulated input signals are combined after making the phases (time deviation) correspond to each other. Therefore, a received signal having a high gain is obtained. A set of these de-spreading circuits and demodulation circuits are referred to conventionally as a rake finger, and the number of the fingers are selected according to the spread of the delay profile on the radio path of the system.
Now, an antenna of a mobile station has directivity as a result of the blocking of radio waves by the body of the operator, because the receiver of a mobile station, such as a portable telephone or a mobile telephone, is used by putting the receiver against one""s ear, that is, by holding the unit near the head of the operator.
As described in the Japanese patent Laid-open print No. 7-231278 official gazette, in case of rake receiving by de-spreading multipath signals which are decomposed to several paths, to obtain a high path diversity gain, it is assumed that the difference in the delay time of a receiving wave corresponding to the difference between path lengths is dispersed with a sufficient spread. That is, it is necessary to employ receive paths whose radio path lengths are greatly different, and for that, it is critical that waves that come from various different directions are received.
However, in case of using a receiver positioned near the head of the operator, the gain of the rake receiving cannot be high because the directions of the received waves are limited, and the extent of the delay spread of the received waves caused by the difference in the length of their paths is not sufficient. Thus, diversity systems are conventionally used in such a case. In a diversity system, spread spectrum signals are received by two antennas arranged so that the correlation between the received waves is almost zero, and the signal that has the higher receiving level is selected.
A diversity system for reducing the influence of the multipath effect and improve the S/N ratio is disclosed in the Japanese patent Laid-open print No. 7-87057 official gazette. In this system, one of the spread spectrum signals received by the two antennas is delayed and combined with another signal. That is, the spread spectrum signal received by one of the antennas is delayed for over one chip length (exp. four chip lengths), and the signals from the two antennas are combined to prevent interference by the two received signals. After the combined signals are de-spread, the de-spreaded signal and its delayed sinal that is delayed for said chip length are combined to obtain a high gain.
However, there is no specific disclosure of a way of delaying a high frequency signal. The delay time for several chips is several microseconds in the conventional system. When using a general delay line for delaying a high frequency signal, the delay line becomes very long to obtain a delay of several microseconds, and this becomes a problem in a mobile station in which miniaturization is required.
The object of the present invention is to avoid a reduction of the circuit quality (speech quality) when the mobile station of a spread spectrum communication system is located near the head of a human operator and avoid the need for an increase in the size of the mobile station.
The foregoing object of the present invention can be achieved in the following way.
A rake receiving circuit is provided in a mobile station using a spread spectrum communication technique. The rake receiving circuit separates a multipath signal which has been spread by spread spectrum modulation using a spreading code into several path signals whose propagation delay time is different using plural de-spreading means, demodulates each separated path signal, adjusts the time base of each demodulated path signal, and then combines the signals. Plural antennas are arranged so that the cross-correlation of a received wave becomes small. Signals received at these plural antennas are input and delayed by the delay means so that the time difference of each input signal is at least more than 1 chip time of the spreading code. Each signal outputted from the delay means is combined by a composing means. The output signal of this composing means is input to the rake receiving circuit.
By doing this, the signals received at the plural antennas are delayed for each received signal by a delay means, and the mutual time differences of each received signal are dispersed on the time base by more than 1 chip time of the spreading code. As a result, the correlation between signals received at each antenna becomes lower or zero by the delay dispersal to a time difference of more than 1 chip time of the spreading code.
Therefore, there is no damping or the signal amplitude by interference even if plural signals received by the respective antennas are simply added and combined at the high frequency level, and so delayed spreading signals suitable for a rake receiving circuit can be combined. Therefore, even if the mobile station is used near the head of a human operator, combined signals with a high diversity gain, which is a merit of the rake receiving, can be obtained, and a reduction of the line quality can be suppressed.
A well-known surface acoustic wave filter is used as a delay means. This filter converts a received signal into a surface acoustic wave signal, that is a mechanical vibration, and delays the signal. That is, in this case, the delay time of a spreading chip unit is very short, while it becomes a high magnitude, when a delay line is used. Thus, using the surface acoustic wave filter can miniaturize the delay circuit.